1. Field of the Invention
The present invention relates to an oxidation-resistant ferritic stainless steel, a method of manufacturing the steel, and a fuel cell interconnector using the steel, and more particularly, to an oxidation-resistant ferritic stainless steel having a Cu-containing spinel-structured oxide formed on a base material, a method of manufacturing the steel, and a fuel cell connector using the steel.
2. Description of the Related Art
Ferritic stainless steels, which generally contain 11% by weight or more of Cr, are cheaper than austenitic stainless steels and stress corrosion cracking due to chlorides does not occur in ferritic stainless steels. Due to these characteristics, demands for ferritic stainless steels have been gradually increased.
A ferritic stainless steel used in a high temperature environment, for example, as a material for boilers and pipes of power plants, exhaust pipes of vehicles, or fuel cell interconnectors, is required to have high thermal resistance and excellent oxidation resistance.
Particularly, when a ferritic stainless steel is used for a fuel cell interconnector, the ferritic stainless steel serves as a separator that separates fuel supplied for a fuel cell from air and serves as a connector between unit cells, thus characteristics such as a high electrical conductivity, an excellent oxidation resistance at an operation temperature, a coefficient of thermal expansion similar to other parts of the fuel cell, and a low cost are required. Conventionally, ceramic materials such as graphite, Cr-5Fe-1Y2O3-doped LaCrO3, or the like, which have excellent stability at a high temperature, were used as materials for a fuel cell interconnector. However, metallic materials are mainly considered due to processability problems. In a case of SOFC, a Ni-based superalloy such as inconel has been often used, but since commercial use of SOFC is difficult because of its high price, active studies have recently been done on use of a ferritic stainless steel.
In a conventional art, oxidation resistance of ferritic stainless steels has been improved by reducing the content of impurities such as C, N, and O and adding alloy elements forming a stable oxide layer such as Cr, Ni, Co, Zr, and rare earth metals.
In KR 2006-0096989, a method for making a ferritic stainless steel article having an oxidation resistant surface includes providing a ferritic stainless steel comprising aluminum, at least one rare earth metal and 16 to less than 30 wt % chromium (here, the total weight of rare earth metals is greater than 0.02 wt %). At least one surface of the ferritic stainless steel is modified so that, when subjected to an oxidizing atmosphere at high temperature, the modified surface develops an electrically conductive, aluminum-rich, oxidation resistant oxide scale comprising chromium and iron and a having a hematite structure differing from Fe2O3, alpha Cr2O3 and alpha Al2O3 to increase high thermal resistance of a ferritic stainless steel is disclosed.
In KR 2005-0093421, a separator for a fuel cell that comprises a metal substrate that contains at least one metal element M and a surface layer formed on a surface of the metal substrate and that contains at least one conductive oxide represented by LaMxO3 (wherein x=0 to 1) to provide a separator for a fuel cell that has an excellent electrical conductivity and significantly improved anti-corrosiveness is disclosed.
In KR 2010-0120401, a interconnector for a solid oxide fuel cell including a substrate formed of a ferritic stainless steel, a first coating layer including at least one of Ni and Cu, wherein the first coating layer is formed on the substrate, and a second coating layer including a rare earth metal, wherein the second coating layer is formed on the first coating layer to provide a interconnector for a solid oxide fuel cell that may form an oxide with an excellent conductivity on a surface of the substrate in an oxidation atmosphere at a high temperature is disclosed.
In KR 2011-0094048, a separator material for a fuel cell including a Au—a first composition alloy layer or a Au layer, wherein the first composition is formed of at least one metal selected from the group consisting of Al, Cr, Co, Ni, Cu, Mo, Sn, and Bi; and a middle layer between the alloy layer or the Au layer and the stainless steel layer, wherein the middle layer is consisted of 20 wt % or more of the first composition and 20 to 50 wt % of oxygen. The alloy layer or the Au layer has an area with a thickness of 1 nm or greater from the outermost surface to a lower layer that contains 40 wt % of Au, or an area with a thickness of 3 nm or greater from the outermost surface to a lower layer that contains 10 to 40 wt % of Au, wherein the Au layer has a thickness of 1 nm or greater.
However, although oxidation-resistance of a ferritic stainless steel may be increased since the prior arts stated above adds Mo, Al, and rare earth metals, or applies a coating layer or an alloy layer on a substrate, intensity of the ferritic stainless steel may be weakened or processability may be deteriorated, and manufacturing costs of the ferritic stainless steel may increase due to the addition of oxidation resistant elements.
Meanwhile, when a stainless steel with a high content of Cr is used as an interconnector for a fuel cell, Cr sticks to a cathode of the fuel cell as Cr is volatilized while operating the fuel cell, and thus performance of the fuel cell is degraded. Conventionally this problem has been solved by using a coating method or the like, but the coating method may not be appropriate due to a feature of an interconnector that may result a complicate shape in a flow channel design procedure.